The present invention relates in particular to a process for generating an H2/CO syngas by steam methane reforming (SMR) (or steam reforming with other hydrocarbon feedstocks such as naphtha, fuel oil, methanol, etc.) or by autothermal reforming (ATR).
Syngas production is in general associated with steam production which itself involves a deaeration process.
During syngas production, a gas mixture essentially containing hydrogen, carbon monoxide, carbon dioxide, water and methane is in fact generated, this mixture being called syngas.
The syngas may be produced by various methods. Mention may especially be made of steam methane reforming or SMR, autothermal reforming or ATR, non-catalytic or catalytic partial oxidation or POX, and reforming using a membrane catalytic reformer, these various syngas production methods being able to be combined with one another and/or combined with gas heated reforming or GHR. These methods may also be supplemented with a shift reaction intended to reduce the production of CO in favor of greater production of H2 and CO2. Whatever the method used, the syngas is produced at high temperature, generally between 600 and 1000° C.
During syngas generation, various types of reaction are involved, always at high temperature. Certain reactions are endothermic and require large amounts of heat, while others are exothermic, so that large amounts of heat are produced via the output fluids (syngas, flue gas).
Large amounts of heat must thus be removed both from the product obtained, i.e. the syngas, so as to lower its temperature sufficiently for it to be able to be treated, and from the combustion gas.
To utilize this available heat and to meet existing steam requirements on the site, or nearby, a syngas production unit is frequently combined with delivery of steam in variable amount as by-product.
This steam comes at least partly from the water contained in the syngas—also called process condensate—which is recovered and recycled. The steam may also be produced from water external to the process, in general demineralized water. The amount of heat delivered by the process is such that it makes it possible in general to produce a supplement of steam in relation to the requirements of the process. Thus, steam may be exported to one or more customers.
Often customers require high-quality steam, that is to say having low concentrations of impurities, for example dissolved gases such as oxygen, carbon dioxide, ammonia, etc. or liquid contaminants such as methanol, formic acid, amines, etc. This is in particular the case when the steam is intended to be exported away from the process.
In particular, water intended to be supplied to boilers must be deoxygenated in order to eliminate the dissolved oxygen present in amounts of the order of the saturation content (9 to 10 ppm) down to practically zero before it can be used in a boiler. This operation is necessary because of the corrosive behavior of oxygen on the metal components both in boilers and in pipes.
Moreover, during syngas production, side reactions also take place (for example, but not exclusively, during catalyzed steps) which generate impurities such as ammonia, methanol, formic acid and amines, which impurities are soluble in water. The water containing impurities is removed from the produced gas by condensing it, and therefore these impurities are still in the condensed water intended to be recycled. This water thus becomes increasingly contaminated with at least the abovementioned contaminants. These impurities, or at least some of them, may be injurious for reforming catalysts. It is therefore also necessary to purify this water before it is reinjected into the steam generation system.
Therefore, plants producing syngas and optionally steam for exportation conventionally include additional equipment intended for treating the water so as to be able to meet the purity requirements. Among such equipment are in particular the deaerators intended for eliminating the harmful water-soluble gaseous elements, the principle ones being oxygen, carbon dioxide and ammonia dissolved in water.
In a syngas generation unit, the dissolved O2 comes in particular from the demineralized water and the dissolved CO2 comes from the process condensates that are recycled thereinto, but there are also other impurities, and NH3, CH3OH and organic acids sometimes dissolved in the process condensates. The deaeration operation is carried out by stripping the water to be treated countercurrently with clean steam.
Deaerators are known as such, and their operation will be briefly described below in conjunction with FIG. 1.
The dissolved gases are eliminated in strippers. These are in the form of small stripping columns (or domes) provided with trays or packing elements that are installed in the upper portion of the deaerator. The steam used for the stripping is conventionally generated in the deaerator itself, but it may also come from a source external to the deaerator.
The gas mixtures vented from the strippers are generally sent to atmosphere, which becomes polluted therewith. The impurities present in the water and entrained by the steam during stripping are discharged with the vented gas mixtures. Sending these impurities into the air is prejudicial—it is both disagreeable (odors), hazardous for persons passing nearby, and harmful to the environment.